1. Field of the Invention
This invention relates to electroless nickel plating of materials such as copper substrates which do not spontaneously initiate electroless nickel plating without a catalyst. It is especially useful in the electronics industry, including production of electroless nickel/electroless gold tabs or surface mount pads on printed circuit boards, and for use in electroless nickel plating over copper for radiofrequency interference shielding. More particularly, it comprises new and improved compositions for effectively and completely catalyzing the initiation of electroless nickel plating on copper substrates. These novel compositions comprise mixtures of a palladium salt with another Group VIII precious metal salt plus both an acid and an alkali halide salt for catalyzing the initiation of electroless nickel plating.
2. Background and Prior Art
Numerous methods have been used to selectively catalyze copper metal for initiation of electroless nickel plating. Copper is considered a poor or even a non-catalyst for electroless nickel compositions using sodium hypophosphite as the nickel reducing agent. Some method must be used to cause the initial plating of electroless nickel. Once a small amount of electroless nickel has plated, electroless nickel plating will continue without need for further catalyzation of the surface.
Numerous methods are known to initiate electroless plating onto metals which will not spontaneously begin plating. G. G. Gawrilov (in Chemical (Electroless) Nickel Plating, Portcullis Press) gives the following methods: contact with aluminum or iron wire while immersed in the electroless nickel; applying a pulse of electric current when first immersed in the electroless nickel; soaking in a solution of sodium borohydride, dimethylaminoborane, or other organoboron reducing agent immediately before immersion in the electroless nickel; electroplating with a thin layer of electrolytic nickel; using an electroless nickel `strike` bath of high hypophosphite concentration; and immersion in an acid solution of palladium chloride.
The most common catalytic initiation method is the use of a solution of palladium chloride (0.01-0.3 g/l) in hydrochloric acid.
All of these methods suffer from one or more disadvantages. Contact with aluminum or iron wires is useful only for small parts. Printed circuit boards having discrete separate copper pads and circuits cannot be done, as each separated portion of copper will need to be touched. The electric current method and the electrolytic nickel plating methods likewise cannot be used on discontinuous copper sections. If the electric current method is used on large and complex parts with crevices and recessed areas, poor and nonuniform catalysis occurs. Surface treatment with solutions of boron reducing agents can be effective, but these solutions are high in actual usage cost since they rapidly decompose when trace amounts of metal ions are introduced. The boron reducing agents can also desorb from the part and destabilize electroless nickel plating solutions. Use of a very active hypophosphite electroless nickel `strike` is not reliable, and the strike bath quickly decomposes.
The best and most widely used commercial catalysis method is the use of a highly acidic solution of palladium chloride. A typical commercial formulation is Activator 440 (Enthone Inc.). This consists of 4.4 g palladium chloride per liter in 8% hydrochloric acid. Recommended use condition is dilution to 6%, or 275 mg palladium chloride per liter. Hydrochloric acid is added as necessary to control hydrolytic decomposition of the palladium chloride. While effective under normal circumstances, it suffers from several disadvantages, especially when used to catalyze printed circuit boards. Palladium chloride rapidly attacks and forms an immersion deposit on most metals. This deposit does not give a continuous metallic deposit. It forms small catalytic sites which continue to grow as long as the copper is immersed in the catalyst solution. Excess catalysis leads to overactivation problems in the electroless nickel, as excess palladium may not adhere completely. This will contaminate and decompose the electroless nickel, and also cause poor adhesion of the electroless nickel. The operating window for good catalysis is very narrow, requiring precise control of immersion time and palladium concentration. Good rinsing is critical for good results. Consumption and overuse of palladium is very great. Printed circuit boards, especially those having numerous small discrete copper areas, are difficult to uniformly plate with the reliability needed for a commercial process. This problem is especially severe when dealing with boards intended for surface mount applications. These boards have tiny wells produced by a 1-5 mil thick layer of organic solder mask completely surrounding the surface mount tabs. These wells are very difficult to completely rinse and catalyze.
The commercial catalytic olefin oxidation process known as the Wacker process uses acidic palladium chloride solutions. Only ionic palladium is catalytic for this process, and it is reduced to palladium metal during olefin oxidation. A second catalytic cycle is coupled with this process to continuously regenerate ionic palladium. The second catalyst system consisting of air and cupric chloride rapidly redissolves metallic palladium. While not wishing to be bound by theory, it is known that copper ions rapidly build up in any acidic palladium bath used for copper catalysis. Older used baths are less effective than fresh baths, even when the palladium concentration is controlled. It is likely that a complex precipitation-redissolution cycle involving ionic and metallic palladium, ionic and metallic copper, air and acid, all contribute to the difficulties in achieving uniform process control and reproducibility.
A related patent disclosure Ser. No. 07/756,626 filed Sept. 9, 1991, now abandoned, by the present applicants has shown that improved results for catalyzation of copper can be effected by use of mixtures of palladium and a Group VIII precious metal salt in acidic solution. This process does suffer from several disadvantages. The highly acidic catalyst is corrosive to the substrate, so immersion times must be controlled and of short duration to prevent overdeposition of catalyst on the surface. The copper which is dissolved in the catalyst solution can attack and redissolve the deposited palladium metal, leading to poor adhesion. The pH of the catalyst bath must be kept low with hydrochloric acid, to prevent catalyst decomposition due to spontaneous hydrolysis of the precious metal salts. Rinsing is difficult due to the rapid hydrolysis of the catalyst as the pH is increased.